Several popular high temperature strain gages employ various alloys of iron, chromium and aluminum (FeCrAl) whose "apparent strain", output due to resistance change versus temperature when no thermal or mechanical strain is present, is the dominant output for these type strain gages. These gages have been typically installed as one arm of a Wheatstone bridge and when the bonded strain gage is strained (elongated or compressed) the gage resistance changes as a function of the strain. This strain is then converted into an output signal representative of the magnitude of the strain. The indicated output due to apparent strain resulting from a temperature change is usually far greater than the resistance change generated due to mechanical strains. Additionally, the resistance change is non-linear, non-repeatable, and drifts with time at elevated temperatures.
Various types of strain gages are known which compensate for apparent strain caused by high temperatures. One apparatus incorporates temperature sensitive wire in the same arm of the bridge circuit as the active strain gage or in an adjacent arm of the bridge circuit. This apparatus corrects the thermal output but only at one temperature due to the non-linear thermal coefficient of resistance of the high temperature alloyed wire from which high temperature strain gages are constructed. Another technique involves installing a strain gage identical to the active strain gage on a test block and wiring the adjacent arm to the active gage while attempting to subject both gages to the same temperature. Maintaining both gages at the same temperature is virtually impossible so that the apparent strain induced by temperature as read by the test block gage and that induced in the active gage are not the same.
It is accordingly an object of the present invention to provide a compensated strain gage which can correct temperature-induced errors associated with gages which can function at temperatures above 370.degree. C.
It is another object of the present invention to provide a compensated strain gage in which the compensating gage is maintained at precisely the same temperature as the active strain gage.
It is another object of the present invention to provide a compensating gage which does not respond to mechanical strain.
It is another object of the present invention to provide a compensated strain gage which is effective over a range of temperatures.
It is another object of the present invention to achieve the foregoing objects in a simple manner.
Additional objects and advantages of the present invention are apparent from the drawings and specification which follow.